Document 'Continue As New' pattern in Laravel Workflow

Added documentation for the 'Continue As New' pattern in Laravel Workflow, including usage examples and benefits.

Author: rmcdaniel

docs/features/continue-as-new.md

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+---
+sidebar_position: 12
+---
+
+# Continue As New
+
+Laravel Workflow supports the **Continue As New** pattern, allowing a running workflow to restart itself with new arguments.
+This is useful when you need to:
+
+* Prevent unbounded workflow history growth.
+* Model iterative loops or recursive workflows.
+* Split long-running workflows into smaller, manageable executions while preserving continuity.
+
+## Using `continueAsNew`
+
+To restart a workflow as new, call the static method `WorkflowStub::continueAsNew(...)` from within the workflow’s `execute()` method.
+
+```php
+use Workflow\ActivityStub;
+use Workflow\Workflow;
+use Workflow\WorkflowStub;
+
+class CounterWorkflow extends Workflow
+{
+    public function execute(int $count = 0, int $max = 3)
+    {
+        $result = yield ActivityStub::make(CountActivity::class, $count);
+
+        if ($count >= $max) {
+            return 'workflow_' . $result;
+        }
+
+        return yield WorkflowStub::continueAsNew($count + 1, $max);
+    }
+}
+```
+
+In this example:
+
+* The workflow executes an activity each iteration.
+* If the maximum count has not been reached, it continues as new with incremented arguments.
+* The final result is returned only when the loop completes.